I can't install anything using Firefox

I switched to Mozilla from Chrome temporarily and Mozilla did not have Adobe Flash Player and i could not watch videos, so I tried to download it and it'll get to probably 99% and then stop. It does the same thing when i tried to install Chrome and Origin and I'm sure any other program. I have McAfee virus protection and I read that it might be stopping it from installing so i turned off the firewall and real-time scanning to try to install the programs again and it still did not work. I'm not so sure this is much of a Firefox issue and more of a computer issue but I'm really desperate to fix this issue.
edit: I tried to update the browser and it said the reason it could not update was "The integrity of the update could not be verified"

I switched to Mozilla from Chrome temporarily and Mozilla did not have Adobe Flash Player and i could not watch videos, so I tried to download it and it'll get to probably 99% and then stop. It does the same thing when i tried to install Chrome and Origin and I'm sure any other program. I have McAfee virus protection and I read that it might be stopping it from installing so i turned off the firewall and real-time scanning to try to install the programs again and it still did not work. I'm not so sure this is much of a Firefox issue and more of a computer issue but I'm really desperate to fix this issue.
edit: I tried to update the browser and it said the reason it could not update was "The integrity of the update could not be verified"

In order to change your Firefox Configuration please do the following steps :# In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear.

Click I'll be careful, I promise! to continue to the about:config page.

look for scanwhendone which is a scanner that checks downloads before they complete.

In the past there were two things:
# Scanwhendone turn off temporarily:
In order to change your Firefox Configuration please do the following steps :# In the [[Location bar autocomplete|Location bar]], type '''about:config''' and press '''Enter'''. The about:config "''This might void your warranty!''" warning page may appear.
# Click '''I'll be careful, I promise!''' to continue to the about:config page.
#look for scanwhendone which is a scanner that checks downloads before they complete.
2. #[[What to do if you can't download or save files]] and [https://support.mozilla.org/en-US/kb/update-failed-error-message-when-updating Update Failed Error Message]
I hope this helps.